This invention relates to an internal combustion engine, and in particular, to a two-phase fuel system and method for reducing undesirable emissions.
Engine technologies have advanced to the point that a properly tuned engine will operate with minimal emissions. When an engine is utilized for automotive transportation, the variations in operating conditions and power requirements require various compromises to achieve both fuel efficiency and low emissions. Where an engine is operating under less than optimum conditions, various scavenging and recirculating systems are utilized to complete the combustion process of partially burned fuel. Ultimately, discharged exhaust gases must pass through a catalytic converter where any remaining hydrocarbons that have not been previously combusted are oxidized before final discharge to the atmosphere.
The part of an engine operating cycle that generates a substantial quantity of undesirable emissions is the period of start-up. During an engine start-up, particularly a cold start-up, the gas scavenging systems and thermal conditions that are designed for optimum operation are not yet functioning. A cold catalytic converter is ineffective in eliminating unburned hydrocarbons that pass through the exhaust system. In aspirated liquid fuel engines, cold liquid fuel is sprayed into the combustion chamber intake, wherein several cycles may occur prior to ignition and start-up. Once started, the excess fuel introduced to initiate start-up may burn incompletely and at reduced temperatures in the cold block so that high rate hydrocarbon gases are generated. Furthermore, after ignition and before completion of a warm-up period, a rich fuel mixture is provided to improve the fuel flammability. This further contributes to excessive undesirable emissions during the period of start-up. During the start-up period, the cold catalatic converter is incapable of finally combusting the start-up gases that are exhausted in the initial phases of engine operation. Before the catalytic converter is heated sufficiently to activate the final oxidation of hydrocarbon exhaust gases. The unburned gases pass to the atmosphere.
The two-phase fuel system of this invention addresses the problem of engine pollution during start-up and incorporates an auxiliary fuel subsystem for supplying and delivering a gaseous fuel to the engine during the critical period of engine start-up. Once the engine is started, and the engine approaches a sufficient temperature that emissions from combustion of liquid fuel can be controlled, the supply of gaseous fuel is phased out, and the supply of liquid fuel is concurrently phased in. In this manner, any fuel that escapes the activated scavenging systems of the engine, are eliminated by the now heated and functioning catalatic converter.
Since the storage of a gaseous fuel, even in liquified form requires significant auxiliary space, it is preferred that the phase of engine start-up that utilize gaseous fuel be minimized. It is preferred that the use of gaseous fuel during engine start-up be supplemented by gassified liquid fuel drawn from the liquid fuel tank. This feature extends the useful term of a gaseous fuel storage canister and/or reduces the size of the canister required for a desired number of cold-engine starts.
The two-phase fuel system of this invention can be incorporated in new engine systems or added as a retro-fit to existing engine systems.